Shanghai, July 25 - China will tighten its control over the Internet as the Olympic Games approach by ordering Chinese Web sites to censor certain content, Interfax sources with several online community and blogging platforms said this week.

"We received notices from the Public Security Bureau and the Propaganda Department this week, asking us to closely watch for 'unhealthy' information. We have added many key words into our supervision system to watch for such information," said a source who works for an online community platform under a state-owned newspaper.

The Chinese government previously promised that it would give greater freedom to its media in the run-up to the Olympics. Though it has unblocked many foreign Web sites, especially in Beijing, Chinese-language sites have come under greater scrutiny.

"In the past, we generally watched for posts that contain Party leaders' names, pornography or violent content. Starting this week, more words have become sensitive," the source said.

The source said that some posts containing sensitive key words will be deleted. The key words include Olympic-related themes, names of Chinese nationalities or ethnic groups and comments about terrorism.

When contacted by Interfax, several other sources working for online communities and blogs in Beijing and Shanghai confirmed that Internet censorship has tightened due to the Olympic Games.

"This month through to late August is a sensitive and busy period for us," a source working for a privately-owned online community said. "Several departments will work to supervise sensitive information, including community service operators, public security bureaus, propaganda departments and telecom network operators that manage servers."

"If we lose control of the sensitive information, our server will be shut down by telecom network operators," they said.

Earlier this month, some online communities, including bbs.hangzhou.com.cn, youcha.net and actcn.net, posted announcements saying that any "unhealthy" comments that could harm the Olympics will be blocked, and offending posters' IP addresses will be given to the Public Security Bureau.

Shahe99.com, a Guangzhou-based online community, went so far as to announce on July 3 that it will forbid users from discussing any political news during the Olympics. A section of the forum called "News from around China" will be closed from July 3 until the end of the Olympics.

Sohoxiaobao.com, a popular blog site, announced on July 18 that it was temporarily shut down because bloggers posted sensitive articles in their blogs. "We hope bloggers will avoid sensitive topics so as to protect Sohoxiaobao, especially during the Olympics," the company said in an announcement.